The BATT Program

Gao Liu at LBNL has developed a new kind of composite anode based on silicon that can absorb eight times the lithium of current Li-ion batteries and maintains a high capacity of 2100 mAh/g in Si after 650 cycles. The key to such improved cyclability is a tailored polymer with dual functionality: it conducts electricity and binds closely to silicon particles as they undergo more than a 300% volume change during the lithiation process (Figure 1).

Figure 1. (Left) Traditionally, composite anodes using silicon (blue spheres) contain a polymer binder such as PVDF (light brown) and a conductive carbon additive (dark brown spheres). Silicon swells and shrinks while acquiring and releasing lithium ions, and this repeated volume change eventually breaks electrical contacts to the carbon particles. (Right) The new Berkeley Lab polymer (purple) is itself conductive and continues to bind tightly to the silicon particles despite repeated volume change, thus maintaining conductivity in the electrode over many hundreds of cycles.

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Understanding volume change and conductivity in Si nanostructures for Li-ion anodes

Silicon is a promising next-generation anode material for high-energy lithium-ion batteries due to its high specific capacity, which is theoretically 10 times greater than graphite.  However, its cycle life is limited due to volume expansion and fracture during lithium reaction.  This degradation of the Si results in loss of electrical connection and pulverization of the electrode. Several fundamental studies still need to be conducted to develop viable Si electrodes for batteries. Yi Cui’s group at Stanford University is working on understanding the properties of various Si nanostructures and is designing new ones based on particles and wires that target improving Si cyclability.

Figure 1. (Left) TEM image of interconnected hollow Si spheres. (Right) SEM image of hollow Si nanospheres scraped using a sharp razor blade, revealing the interior empty space in the spheres and their interconnected shells.

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Predicting the Properties of Ionic Liquids from a Polarizable Force Field in Molecular Dynamics Simulations

Oleg Borodin at the Army Research Laboratory has developed and validated a polarizable force field for a wide class of ionic liquids (ILs), which are being explored as additives to lithium-battery electrolytes for improved stability. The developed force field for a wide range of anions shown in Figure 1 will serve as a starting point for molecular dynamics simulations (MD) of liquid electrolytes doped with variety of conventional and novel lithium salts.  This force field, when used in MD simulations, is a tool for predicting thermodynamic and transport properties of conceptual ILs and IL-solvent mixtures and may shed light on structure-property relationships. Using predictions of promising properties to direct materials design can accelerate the development of ILs suitable for battery applications.

Figure 1. A representative set of cations and anions for which the many-body polarizable force field has been developed.

High-Rate NMC Cathodes Achieved with Carbon Nanotube Additive

Through a collaborative effort, the Dillon Group at NREL and the Whittingham Group at SUNY -Binghamton have enhanced the conductivity of NMC cathodes to improve their rate capability in Li-ion batteries. These layered cathodes usually suffer from poor electrical conductivity and capacity fade at high charge/discharge rates. To mitigate these problems, the researchers have incorporated single-wall carbon nanotubes (SWNTs) into the NMC cathodes (LiNi_0.4Mn_0.4Co_0.2O₂). The resultant composite cathodes exhibit stable high-rate capacities, ~130 mAh/g at 5C and nearly 120 mAh/g at 10C for over 500 cycles, which are significantly higher than those achieved with conventional NMC cathodes.

Electrochemical activity of tin depends on surface crystal orientation

Figure 1. Cyclic voltammograms of single crystal Sn(100) and Sn(001) electrodes in 1 M LiPF6, EC:DEC (1:2, w:w), 1 mV/s

The Kostecki Group at Lawrence Berkeley National Laboratory aims to understand how the electrochemical response of polycrystalline metal anodes depends on local surface structure.  They have found that different crystal structures at surfaces of Sn induce different interfacial reactions.  These studies shed light on how electrode surface orientation affects electrolyte reduction processes and the formation of a stable solid electrolyte interface (SEI) layer during battery formation.
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Real-time Observation of Morphology Changes in SiO_x Anodes for Lithium-ion Batteries

The Zaghib Group at Hydro-Québec has used in situ SEM to see SiO_x particles grow and shrink during cycling.  SiO_x is a promising anode material for Li-ion batteries due to a high theoretical specific capacity of 1338 mAh/g and less volume change than Si upon charge-discharge. Analysis of the morphology changes in SiO_x particles provides insight into the failure mode associated with capacity fade on cycling.
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Designing Better Electrolyte Components for Lithium-Ion Batteries 

Researchers at North Carolina State University are cooking up next-generation electrolytes for lithium-ion batteries, and their recipe calls for more salt. The Ionic Liquids and Electrolytes for Energy Technologies (ILEET) Laboratory, run by Professor Wesley Henderson, creates and characterizes new lithium salts and ionic liquids to formulate electrolytes with a wider range of operating temperatures and voltages. Electrolytes in current Li-ion batteries are limited by poor low temperature performance (< -10°C) and decompose at elevated temperatures (>55°C), potentially causing a fire hazard. To circumvent these issues, Henderson is developing new electrolyte components based on organoborates related to bis(oxalato)borate (BOB^-) and cyanocarbanions. These anions are first synthesized as lithium salts, and then scaled up to form ionic liquids, which are salts that remain liquid even at room temperature or below and have low volatility. 

Figure 1. The chemical structure of LiBOB (left) and its crystal structure (right). The elemental color key is: Li (purple), O (red), B (tan), and C (grey). Ref: Zavalij, et al. Acta Crystallogr. 2003, B59, 753-759.

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Lithium Diffusion Pathways in Graphitic Carbon Anodes

Figure 1. Graphic showing faster lithium-ion diffusion in the direction parallel to the graphene planes.

Graphitic carbon is widely used as an anode material in lithium-ion batteries. For high-power applications such as hybrid electric vehicles, however, prolonged cycling at high rates can damage a graphite anode and lead to plating of lithium metal, thereby decreasing the lifetime and capacity of the battery. Elucidating lithium diffusion pathways in graphite provides insight into why these anodes are limited by modest charge/discharge rates. The Persson and Kostecki Groups, in collaboration with other BATT investigators, have quantified lithium-ion diffusivity as a function of transport direction in graphite anodes.[1] Electrochemical experiments combined with first-principles calculations indicate that lithium diffusion in graphite is several orders of magnitude faster in the direction parallel, as opposed to perpendicular, to the graphene plane. These results provide guidelines for designing graphite anodes with preferential orientation for higher rate capability, which translates to faster charging batteries.

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Figure 1. Transmission electron microscopy image of a carbon nanotube covered with Si nanoclusters.

Two Approaches toward Silicon-Carbon Composites as High-Capacity Anodes for Lithium-Ion Batteries

The Kumta Lab is developing low-cost methods for producing nanoscale silicon-carbon composites as lithium-ion anodes.  These heterostructures comprise nanocrystalline or amorphous Si and a variety of carbon precursors.  The Si provides high capacity while the graphitic and disordered carbon acts as an electrically-conductive matrix as well as a mechanically compliant phase.  The Kumta Lab’s multi-pronged approach towards anode fabrication includes chemical vapor deposition (CVD) and high-energy mechanical milling (HEMM).

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Figure 1. Oxygen released versus temperature for MPO4 (M = Mn, Fe)

Thermal Stability of Delithiated LiMnPO₄ Cathodes Determined from First Principles Phase Diagrams

The Ceder Group has calculated that delithiated LiMnPO₄ cathodes decompose at a much lower temperature than delithiated LiFePO₄ cathodes.  These results confirm experimental findings by Chen and Richardson [1] and Kim et al. [2] that delithiated LiMnPO₄ decomposes at relatively low temperatures of around 150–200 °C, releasing O₂ and heat that might ignite the organic electrolytes used in Li-ion batteries. This is in stark contrast to delithiated LiFePO₄, which is known to decompose at much higher temperatures of around 500 °C. Therefore, despite having a higher energy density (at 4.1 V vs. Li/Li⁺) than LiFePO₄ (at 3.5 V vs. Li/Li⁺), the olivine LiMnPO₄ poses lower safety as a cathode material.

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Lithium Nitride for Prelithiating Anodes to Reduce Capacity Losses in Li-Ion Batteries 

The Richardson Lab has developed a new method for prelithiating anodes to compensate for the first-cycle capacity loss associated with conventional lithium-ion batteries.  The prelithiated anodes supply lithium for the irreversible reactions that occur upon initial charging, such as decomposition of the electrolyte to form a solid electrolyte interphase (SEI) layer on the anode.  Since lithium ions from the cathode are not consumed in SEI formation, the battery capacity is retained. 

Figure 1. Early charge/discharge cycles of untreated (left) and prelithiated (right) Sn/C anodes.

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A Reactive Molecular Dynamics Simulation Study of Single-Electron Reduction Pathways

Figure 1. Stable product of ethylene carbonate decomposition as predicted by RMD simulations of reactions between closed and open forms of the EC radical anion.

The Smith Group, at the University of Utah, has identified reaction products of the single-electron reduction of ethylene carbonate (EC), an important component in lithium-ion battery electrolytes.  Reactive force field (ReaxFF) simulations were used to discover the fate of EC, which provides insight into the structure of the solid electrolyte interphase (SEI) – a thin film covering carbon anodes that limits the charge/discharge rate.  Reactive force fields allow for the making/breaking of chemical bonds during classical molecular dynamics simulations.  Because electronic degrees of freedom are treated in a simplified manner, these reactive molecular dynamics (RMD) simulations allow for the study of chemical reactions in much larger collections of molecules over much longer times than can be studied using ab initio methods.

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Figure 1. Cyclability of lithium nickel manganese oxide and the Fe-substituted cathodes.

Passivation of Spinel Cathode Surface through Self-Segregation of Iron

The Manthiram Lab has developed an iron-doped, high-voltage cathode based on lithium nickel manganese oxide that results in extended cyclability and improved electrochemical performance.  The cathode’s spinel structure enables 3-D Li⁺-ion diffusion and direct metal-metal interaction across the shared octahedral edges, which supports high power capability for HEV and PHEV applications at high operating voltages (> 4.3 V vs. Li/Li⁺). The iron-enrichment on the cathode surface prevents surface-electrolyte instability at these high operating voltages.

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New Material for High-Voltage Anodes in Lithium-Ion Batteries

The Goodenough Lab has synthesized a new anode material that can operate at voltages between 1.0 and 1.6 V vs. Li⁺/Li⁰.  This TiNb₂O₇ (TNO) anode joins the ranks of the spinel lithium titanate (Li₄Ti₅O₁₂) in being an anode material that operates within the window of thermodynamic stability for an organic-liquid carbonate electrolyte. Existing lithium-ion batteries have carbon anodes that operate at voltages below this window of stability, thereby resulting in decomposition of the electrolyte and formation of a thin passivating film on the anode surface.  This solid electrolyte interphase (SEI) layer traps Li⁺ ions, causing the battery to irreversibly lose capacity on the first charge; it also limits the charge/discharge rate. Since the TNO anode does not form an SEI layer, it does not rob capacity from the cathode and is capable of a fast battery charge.

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